The Rail, Tram and Bus Union is pushing for the same 17 per cent pay deal given to train drivers last year but that's been rejected by Metro.

The union says the potential action could bring Melbourne's train network to a halt for 48 hours.

Opposition Leader Matthew Guy on Thursday said the pay claim was significant and the Labor government needed to stand up to the union.

"The requests are very big," Mr Guy told reporters.

"What we are seeing is a message sent to every union in this state that the way to do business is to threaten strikes."

The Rail, Tram and Bus Union says the protected action will potentially cover the full range of Metro's infrastructure maintenance workers - including those working to resolve faults that impact on train operations.

"While we would have preferred to avoid the path of industrial action, the contempt with which Metro is treating our members has left us with few other options," RTBU secretary Luba Grigorovitch said in a statement.

Public Transport Minister Jacinta Allan said negotiations were continuing and Metro had advised the government that there were "no threats to the operation of the network".